Note: Descriptions are shown in the official language in which they were submitted.
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WIRELESS PRIVATE BRANCH EXCHANGE
RELATED APPLICATIONS
This application is a continuation-in-part of U.S. Ser. No. 09/049,606,
pending, which is a continuation of U.S. Ser. No. 08/434,598, now U.S. Pat.
No.
5,734,979, incorporated herein by reference.
This application claims priority to U.S. Prov. No. 60/071,075 filed on
January 15, 1998.
FIELD
The present invention relates to a wireless private branch exchange. In
particular, the invention relates to a system and method to enable private
branch
exchanges to communicate with external telephone networks using a wireless
communication network, to increase communication capacity of the private
branch
exchange in a cost efficient manner.
BACKGROUND
With the ever increasing importance of communication services, both speech
and data, there is a rapidly growing need for providing economically cost
efficient
means of communication. The conventional public telephone service, usually
known
as public switched telephone network (PSTN) or wireline telephony, remains the
predominant communication network used by millions of users around the world.
Wireline telephony allows two users to communicate with each other only if
there is
a physical wire connection between the two users. The necessity of a physical
connection between users imposes several restrictions on users of PSTN
systems.
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First, the capacity of users that a PSTN can service at a given time is
restricted by the number of physical lines existing between users. Hence, the
capacity of a PSTN can be increased only by installing/laying new lines to
support
additional users -- a solution that is very expensive and time consuming. As a
result, the capacity of a PSTN is usually capped by economic considerations.
It
would therefore be highly desirable to have a communication network whose
capacity can be increased in a cost efficient manner.
Another problem with existing PSTN systems is the manner in which new
lines are laid, usually below the ground. This involves extensive digging of
roads
and property. In many instances, this is not a viable option. This is
especially true
in populated urban areas like cities and suburbs where the demand for
communication services is very high but there is very little land available
for laying
new PSTN lines. Furthermore, since PSTN lines stretch out over long distances
between users, their maintenance is very expensive and time consuming. Thus,
there is a need for a communication system which not only increases the
communication capacity but is also easy to install and maintain.
The above-mentioned problems and restrictions of wired PSTN systems have
a major impact on commercial establishments such as offices and hotels which
require large communication capacity. A majority of these commercial
establishments use private branch exchanges (PBXs) to handle their call
processing
needs. A PBX is generally a telephone exchange serving a single organization,
for example, an office building having a switchboard and associated equipment,
usually located on the user's premises. A PBX is generally connected to a
plurality
of private internal telephone users (called terminals) located in a typical
office
environment. The PBX provides for internal call switching between one or more
of
these terminals. The PBX is also connected to external telephone networks via
PSTN lines. The PBX uses the PSTN lines to perform call routing between a PBX
terminal and an external telephone network terminal.
As the commercial establishment served by the PBX grows in size, it
frequently becomes desirable to increase the external communication capacity
of the
PBX at a reasonable expense. However, since the PBX is linked to fixed
capacity
physical PSTN lines, an increase in capacity is possible only by installing
additional
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PSTN lines. As mentioned earlier, in most circumstances this option is
prohibitively
expensive and impractical. Consequently, the external calling capacity of
these
PBXs is forcibly limited to the number of existing PSTN physical links. It
would
therefore be desirable to provide a system for increasing the capacity of
existing
PBXs in a cost efficient and practical manner.
The advent of wireless telephony (or mobile communication) technology has
sought to solve some of the restrictions posed by wired PSTN networks by
decoupling the telephone from its wires. In a wireless communication network,
for
example GSM, the telephone is substituted by a mobile station (MS) or a
handset.
The geographical area serviced by the cellular network is divided in cells --
each cell
being served by a base transceiver station (BTS). Information is exchanged
between
the mobile station and the BTS over a radio interface. The BTS in turn
communicates with a mobile services switching center (MSC) which performs the
basic switching function in the wireless network. The MSC also acts as a
gateway
between the wireless network and other external communication networks --
performing interconnectivity and switching functions between GSM and the other
external telephone networks.
Although wireless telephony removes the restriction of wires, it has its own
share of problems. First, a mobile station is quite expensive when compared to
an
ordinary telephone. Thus, it is not always economically feasible to replace a
telephone with a mobile station. This is especially important for businesses
and
other commercial establishments which have existing PSTN telephone networks
connected to PBXs. Replacing the entire existing PSTN base with a wireless
network is not cost efficient. Second, many functions provided by the private
branch exchange are not supported by wireless telephone services. It would
therefore be desirable if the commercial establishments could retain their
existing
PSTN based PBX systems and also take advantage of the available wireless
technology.
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SUMMARY
The present invention relates to a wireless private branch exchange. In
particular, the invention relates to a system and method to enable a private
branch
exchange to increase its capacity in a cost efficient manner by communicating
with
external telephone networks using a wireless communication network. Exemplary
embodiments are provided for use with the Global Systems for Mobile
Communication {GSM) protocol and can be applied to other cellular
communication
and digital technologies.
A wireless private branch exchange comprises a handset transceiver station
(HTS) coupled to a private branch exchange serving a plurality of terminals.
During
inbound information processing the handset transceiver station receives
inbound
information transmitted by the wireless communication network. The handset
transceiver station decodes the inbound information and performs protocol
conversion from a GSM format to an ISDN format supported by the PBX. The
inbound information is then forwarded to the PBX which routes it to an
appropriate
terminal.
During outbound information processing the handset transceiver station
receives outbound information from the PBX to be routed to an external
telephone
network terminal. The handset transceiver station converts the outbound
information from ISDN format to GSM format. The handset transceiver station
then
encodes the outbound information and transmits it to the wireless
communication
network. The outbound information is then routed by the wireless communication
network to the destination external telephone network terminal.
In one embodiment, the HTS comprises an antenna, a receiver, a transmitter,
a processor and a protocol converter. The receiver receives inbound
information
and the transmitter transmits outbound information. The processor is coupled
to the
receiver and transmitter and is responsible for decoding inbound information
and
encoding outbound information. The protocol converter is responsible for
information format conversions between GSM and ISDN.
In one embodiment of the invention, the PBX is connected to the HTS and
also to an external PSTN network via a physical. link. In this embodiment, the
PBX
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is configured to route the call over the radio interface only if the wired
PSTN
interface is already operating at full capacity. This allows the PBX to make
optimal
use of existing PSTN links and the wireless interface.
In another embodiment of the invention, the concurrent capacity of the
wireless private branch exchange can be increased by using a plurality of
HTSs,
each with their own receive and transmit cycles. The ability to add additional
HTSs
as and when required provides the PBX administrator with an easy and cheap way
to
increase the communication capacity of the PBX.
Advantages of the invention include the ability to increase the communication
capacity of a PBX without the need to install additional PSTN lines.
Furthermore,
in one embodiment of the invention the PBX can make optimal use of existing
PSTN
links and the wireless interface. Since the equipment required for the present
invention is localized at the user's site, it is easy to install and maintain.
The
present invention provides the PBX user with the flexibility to increase the
capacity
of the PBX when required.
BRIEF DESCRIPTION OF THE DRAWINGS
Additional features, aspects and advantages of the present invention will be
more readily apparent from the following detailed description and appended
claims
when taken in conjunction with the drawings, in which:
Figure 1 depicts an exemplary telephone network incorporating the present
invention;
Figure 2 is a flowchart showing the steps involved in inbound information
processing for an embodiment of the invention;
Figure 3 is a flowchart showing the steps involved in outbound information
processing for an embodiment of the invention;
Figure 4 is a block diagram depicting the internal structure of a handset
transceiver station for an embodiment of the invention;
Figure 5 depicts an alternate embodiment of the present invention; and
Figure 6 depicts another alternate embodiment of the present invention.
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DETAILED DESCRIPTION
The present invention relates to a wireless private branch exchange. In
particular, the invention relates to a system and method to enable private
branch
exchanges to increase their communication capacity in a cost efficient manner
by
communicating with external telephone networks using a wireless communication
network. Exemplary embodiments are provided for use with the Global Systems
for
Mobile Communication (GSM) protocol and can be also applied fo other wireless
communication and digital technologies.
The present invention is described with reference to various embodiments
incorporating specific configurations and protocols. Those skilled in the art
will
appreciate that various changes and modifications can be made to the
embodiments
while remaining within the scope of the present invention. For purposes of
this
description, the term information includes voice, data or both.
Referring to Figure 1, there is shown an exemplary telephone network 100
incorporating the present invention. A wireless communication network 102
using
the GSM protocol communicates with a wireless private branch exchange 104 via
the radio interface. The wireless communication network I02 is a conventional
wireless network including a base transceiver station 108 (BTS) connected to a
base
station controller 110 (BSC), which in turn is connected to a mobile services
switching center 112 (MSC). In a typical wireless network, BTS 108 using
antenna
124 communicates via a wireless interface with mobile stations or handsets
which
provide an interface through which end users can access the wireless network.
In
the telephone network 100 depicted in Figure 1, the mobile stations or
handsets are
replaced by the wireless private branch exchange 104. MSC 112 acts as a
gateway
between the wireless communication network 102 and external telephone networks
106. Thus, MSC 112 is responsible for providing interconnectivity between the
wireless network 102 and other external networks 106. An overview of GSM is
described in the U.S. Pat. No. 5,781,582 incorporated herein by reference.
The wireless private branch exchange 104 comprises.an antenna 114, a
handset transceiver station 116 (HTS) and a private branch exchange 118 (PBX)
connected to a plurality of terminals 122. Terminals 122 serviced by PBX 118
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include telephones, fax terminals, and computers. PBX 118 supports functions
performed by conventional PBX systems, such as call routing, call redirection,
call
forwarding, and call waiting. In the embodiment depicted in Figure 1, PBX 118
supports the ISDN protocol and is coupled to HTS 116 via ISDN link 120.
However, in alternate embodiments PBX 118 can also support other telephone
network protocols known to those skilled in the art.
HTS 116 emulates the functions of a typical mobile station. HTS 116
therefore enables PBX terminals 122 to communicate with external telephone
networks 106 over a wireless interface. In general, information processing
performed by the present invention can be classified into inbound information
processing and outbound information processing. Inbound information processing
refers to the steps involved in communicating inbound information from an
external
telephone network terminal to a terminal serviced by PBX 118. Outbound
information processing refers to the steps involved in communicating outbound
information from a terminal connected to PBX 118 to an external telephone
network
terminal.
HTS 116 receives outbound information from PBX 118 and transmits it to
BTS 108 over a radio interface via antenna 114. Similarly, HTS 116 receives
inbound information from BTS 108 over the radio interface via antenna 114 and
forwards the received inbound information to PBX 118. In this manner, PBX 118
can communicate with external telephone networks 106 over a wireless network
without the need for physical PSTN connections between the terminals of PBX
118
and the external telephone network terminals.
Inbound Information Processing
Figure 2 depicts a flowchart 130 showing the steps involved in inbound
information processing for an embodiment of the invention. As shown in Figure
2,
inbound information processing is initiated at step 132 when MSC 112 receives
inbound information from an external telephone network terminal which is to be
forwarded to one or more terminals serviced by PBX 118. 1n order to route the
inbound information to the appropriate BTS, MSC 112 has access to a subscriber
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information database which contains a list of all mobile stations that are
supported
by that particular MSC. As mentioned earlier, in this embodiment of the
invention
the wireless private branch exchange emulates a mobile station. Consequently,
all
terminals 122 of PBX 118 are mapped to the same subscriber entry in the
subscriber
information database. MSC 112 uses destination information contained in the
inbound information to locate the subscriber database entry corresponding to
wireless private branch exchange 104. MSC 1 I2 then routes the inbound
information to BTS 108 which is responsible for communicating with wireless
private branch exchange 104. At step 134, BTS 108 transmits the inbound
information to wireless private branch exchange 104.
At step 136, HTS 116 of wireless private branch exchange 104, receives
inbound information transmitted by BTS I08. At step I38, HTS 116 decodes the
inbound information. Since the GSM information format is different from the
ISDN
information format supported by PBX 118, at step 140, HTS 116 converts the
inbound information from a GSM format to an ISDN format. In one embodiment,
the data format used to transmit data to a PSTN is used to transmit data from
HTS
1 I6 over an ISDN 120. For more information on protocol conversion from GSM to
ISDN refer to M. Mouly, The GSM S~rstem for Mobile Communications, Chapter
3, pg. 140-142 (1992) (International Standard Book Number 2-9507190-0-7).
After
performing the GSM to ISDN protocol conversion, at step 142, HTS I16 forwards
the inbound information to PBX 118. At step 144, PBX 118, uses the destination
information contained in the inbound information to route the inbound
information
to the appropriate terminal(s). This completes the inbound information
processing
process.
Outbound Information Processing
Figure 3 depicts a flowchart 150 showing the steps involved in outbound
information processing for an embodiment of the invention. As shown in Figure
3,
outbound information processing is initiated at step 152 when PBX 118 receives
a
request from one or more of its terminals 122 requesting outbound information
to be
communicated to an external telephone network terminal. At step 154, PBX 118
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forwards the outbound information to HTS 116. As mentioned earlier, the
information received from PBX 118 is in ISDN format which needs to be
converted
to the GSM format before it can be transmitted via the wireless interface. At
step
156, HTS 116 converts the outbound information from ISDN format to GSM
format. In one embodiment, the data format used to transmit data to a PSTN is
used
to transmit data over an ISDN 120 to HTS 116. For more information on ISDN to
GSM protocol conversion, refer to M. Mouly, Th~GSM System for Mobile,.
S~~i~ls, Chapter 3, pg. 140-142 (1992).
At step 158, HTS 116 encodes the outbound information and prepares it for
transmission. At step 160, HTS 116 uses antenna 114 to transmit the encoded
outbound information via the radio interface to BTS 108. At step 162, BTS 108
receives the outbound information transmitted by HTS 116. The outbound
information is then routed via BSC 110 to MSC 112. Depending on the
destination
information contained in the outbound information, in step 164 MSC 112
performs
the necessary inter-networking functions and routes the outbound information
to the
appropriate external telephone network terminal. This concludes the outbound
information processing process.
Figure 4 is a block diagram depicting the internal structure of HTS 116 in
accordance with one embodiment of the present invention. As shown in Figure 4,
HTS 116 comprises an antenna 114, a diplexer 168, a receiver 170, a
transmitter
176, a processor 182 and a protocol converter 184. Antenna 114 is used both
for
receiving inbound information transmitted by BTS 108 and for transmitting
outbound information to BTS 108. A common antenna 184 can be used for both
reception and transmission of information since the transmit frequency band of
the
GSM wireless communication network is disjoint from the receive frequency
band.
For GSM communication, each of the frequency bands occupies approximately
25MHz , with the GSM transmit frequency band approximately in the range of 890-
915MHz and the receive frequency band approximately in the range of 935-
960MHz. Antenna 114 is connected to receiver 170 and transmitter 176 via
diplexer 168.
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Receiver 170 is responsible for receiving the inbound information transmitted
by BTS 108 during inbound information processing. This corresponds to step 136
in
Figure 2. Receiver 170 comprises a receiver synthesizer 172 and a receiver
mixer
174. Receiver synthesizer 172 is programmed to generate the receive frequency
for
HTS 116 which corresponds to the transmit frequency of BTS 108. On receiving
the inbound information using antenna 114, synthesizer 172 provides the
information
frequency to receiver mixer 174, which down-mixes the received signal and
provides the inbound information signal to processor 182.
Transmitter 176 is responsible for transmitting outbound information to BTS
108 during outbound information processing. This corresponds to step 160 in
Figure 3. Transmitter 176 comprises a transmitter synthesizer 178 and a
transmitter
mixer 180. Transmitter synthesizer 178 is programmed to generate the transmit
frequency for HTS 116 which corresponds to the receive frequency of BTS 108.
Transmitter synthesizer 178 provides the frequency to transmitter mixer 180,
which
up-mixes the outbound information signal received from the processor 182 and
provides a radio frequency signal to be transmitted to BTS 108 via antenna
114.
Processor 182 is coupled to receiver 170, transmitter 176 and protocol
converter 184. Processor 182 is responsible for decoding the inbound
information
signal (corresponding to step 138 in Figure 2) and for encoding the outbound
information signal (corresponding to step 158 in Figure 3) to be transmitted
to BTS
108 via the antenna. Protocol converter 184 forms the boundary between the GSM
domain and the ISDN domain supported by PBX 118. Protocol conversion is
required for interconnecting the GSM domain to the ISDN domain. This is
accomplished by protocol converter 184 which converts inbound information from
GSM format to ISDN format (corresponding to step 140 in Figure 2) and outbound
information from ISDN format to GSM format (corresponding to step 156 in
Figure
3). For further details on protocol conversion between ISDN and GSM refer to
M.
Mouly, The GSM System for Mobile Commmications, Chapter 3, pg. 140-142
(1992). It should be apparent to those skilled in the art that the scope of
protocol
converter 184 is not restricted to GSM-ISDN conversions. Conversions between
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other telecommunication protocols known in the art are also within the scope
of the
invention.
Protocol converter 184 is connected to PBX 118 via ISDN link 120.
Protocol converter 184 uses link 120 to forward inbound information to PBX 118
S (corresponding to step 142 in Figure 2) and to receive outbound information
from
PBX 118 (corresponding to step 154 in Figure 3). PBX 118 is connected to a
plurality of terminals 122 including but not restricted to telephones, fax
machines,
and computers. As mentioned earlier PBX 118 supports functions performed by
typical prior art PBX systems, such as call routing, call redirection, call
forwarding,
and call waiting.
Alternate Embodiments
Figure 5 depicts telephone network 190 incorporating an alternate
embodiment of the present invention. As depicted in Figure 5, PBX 118 is not
only
connected to I3TS 116 but is also connected to an external PSTN network 106
via
physical link 192. This setup is typically found in establishments which have
an
already existing PSTN base but want to further increase the capacity of the
PBX
using the wireless communication capability. In this configuration, PBX 118
has a
choice of using wired PSTN interface 192 or the wireless radio interface for
communicating with external telephone networks. Since the cost of
communication
over a wired interface is cheaper than using a wireless interface, it is
desirable that
PBX 118 make use of the wireless interface only if the wired PSTN interface
192 is
operating at full capacity and is unable to handle any more PBX call requests.
In
essence, PBX call requests exceeding the PSTN capacity are routed over the
wireless
interface.
Thus, in this embodiment, PBX 118 is configured to first attempt to make a
connection using the wired PSTN interface. If the PSTN interface is already
operating at full capacity and the call request cannot be successfully
completed, then
PBX 118 routes the connection request via the wireless interface. In this
manner
PBX 118 not only maximizes the PSTN network usage but also allows for
increased
capacity using the wireless interface. This optimal use of the existing wired
PSTN
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services and wireless services translates to substantial cost savings. It
should be
apparent to those skilled in the art that various implementations of this
embodiment
are possible without departing from the true scope of the invention.
Figure 6 depicts a telephone network 194 incorporating another alternate
embodiment of the present invention. As depicted in Figure 6, PBX 118 is
connected to a plurality of HTSs 114-1 to 114-n. For a given HTS, the number
of
PBX terminal connections that can be concurrently maintained over the wireless
interface depends on the number of time slots that make up a GSM frame. A
typical
GSM frame comprises eight time slots. Therefore, a wireless private branch
exchange comprising a single HTS can support up to eight connections
simultaneously over the wireless interface.
A convenient way to increase the number of connections is to increase the
number of handset transceiver stations. As depicted in Figure 6, instead of
one HTS
per wireless private branch exchange, PBX 118 is connected to a plurality of
HTSs
with their own respective receive and transmit frequencies. Since each HTS can
support up to eight connections, the addition of each additional HTS increases
the
capacity of the PBX by eight connections. Thus, the PBX capacity can be easily
increased as and when required by simply increasing the number of HTSs
connected
to PBX 118.
Conclusion
Advantages of the invention include the ability to increase the communication
capacity of a PBX without the need to install additional PSTN lines. This
translates
to substantial savings in costs which would otherwise have to be incurred in
installing additional PSTN lines. It provides commercial establishments with a
cost
efficient way to increase their communication bandwidth.
The invention further reduces costs by maximizing the usage of the cheaper
wired communication interface before routing the connection requests over the
wireless interface. The invention thus allows for optimal use of the wired
PSTN and
wireless radio interfaces.
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As explained above, the capacity of a wireless PBX can be increased quite
easily by connecting additional HTSs to the. PBX. This offers the PBX
administrator
great flexibility in regulating the total capacity of the PBX network.
Unlike prior art PSTN systems where the physical PSTN lines stretch out for
long distances between users, the physical equipment components utilized in
the
present invention are localized at the PBX site. Consequently, installation
and
maintenance of the invention is simple and cheap.
While the present invention has been describe with reference to a few
specific embodiments, the description is illustrative of the invention and is
not to be
construed as limiting the invention. Various modifications may occur which are
obvious to those skilled in the art while remaining within the true spirit and
scope of
the invention as defined by the appended claims.
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